Radio on/off based on hours.

Is there any option/setting on the Airport Extreme to on/off the radio transmission based on the hours of the day?

No

Or to lower down the power

No

if on/off option is not available?

The only on/off option is to unplug and replug the power cable, or use a switched outlet to do the same thing

I believe Apple should consider it in a future update

Since you are speaking to other users here on a user-to-user forum, and there is almost a zero chance of someone from Apple seeing your post, you might want to let Apple know, using the link below.

Apple - AirPort Extreme - Feedback

so I believe is a mandatory task to have this health concept in all the devices.

Then, you will not be able to use your cordless phones....which emit far greater radiation than any WiFi router, or have any electricity at all in your home, since the overhead power lines near your home are emitting constant radiation.

You will also have to figure out how to prevent broadcasts reaching your home (and car) from cell towers, television and radio towers, police and emergency broadcasts, and transmissions from aircraft. There are many others to worry about as well, but you already know about them.

I hope to see this feature request soon

We have seen other occasional requests like this for the past 10 years, since Apple has been selling wireless devices. But, so far, Apple is not listening. Perhaps your feedback using the link above will be the key to convince them to think otherwise. Good luck!

Hopefully, Apple will act on your request.